1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional communication systems transmit information between entities using modulated electromagnetic signals. For example, conventional wireless communication systems include one or more base stations, which may also be referred to as node-Bs or access points or access networks, for providing wireless connectivity to one or more mobile unit, which may also be referred to using terms such as user equipment, subscriber equipment, and access terminals. Modulated signals including data and/or control information may be transmitted between mobile units and base stations over one or more communication channels, which may be formed according to a number of different communication protocols. Exemplary communication protocols include Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA, CDMA2000), and Frequency Division Multiple Access (FDMA). Similar techniques may also be used to transmit modulated signals over wired connections according to protocols such as Digital Subscriber Line (DSL) protocols.
One particular type of FDMA protocol is referred to as Orthogonal Frequency Division Multiplexing (OFDM). An OFDM communication channel may be formed using a baseband signal that includes a number of orthogonal sub-carriers or sub-channels or tones. Signals transmitted on each sub-carrier may be independently modulated using a modulation technique such as quadrature amplitude modulation (QAM) or phase-shift keying (PSK). The composite baseband signal is typically used to modulate a main radio frequency carrier or channel. OFDM may be implemented using efficient fast Fourier transform (FFT) algorithms and the robustness of OFDM communication channels against inter-symbol interference (ISI) makes OFDM a suitable technique for high-data rate systems. Accordingly, OFDM may be used to establish communication channels for a variety of applications, such as wireless local area networks, digital audio/video broadcasting, asymmetric digital subscriber lines (ADSLs), systems based on the IEEE 802.16 WiMAX standard, and the like.
Transitions between different symbols transmitted over an OFDM communication channel typically occur over relatively short time scales. Consequently, the power spectrum of the signals transmitted over the OFDM communication channels may be relatively broad. The relatively slow decay rate of the average power spectral density may result in significant out-of-band energy transmission. The out-of-band energy may reduce the efficiency of the transmitting device and may also cause interference with other devices transmitting in adjacent frequency bands. In a practical OFDM system, such devices that operate according to the IEEE 802.16 WiMAX standard, the out-of-band power should be restricted below a certain level in order not to cause significant interference to other devices in the adjacent frequency bands.
The out-of-band emission may be reduced by filtering the transmitted signal. However, conventional filters for out-of-band emission suppression are complex to implement and may result in significant distortion of the transmitted signal that may reduce or eliminate the orthogonality of the signals. Consequently, the receiver may need to perform additional operations to compensate for the signal distortion introduced by conventional out-of-band emission suppression filters. Alternatively, modulation coding may be used to introduce special correlation into OFDM data symbols in order to shape the spectrum and reduce the effects of inter-carrier interference (ICI). Modulation coding schemes may also be referred to as “self-cancellation” schemes. Self-cancellation techniques may, however, undesirably affect radio resource management when implemented in actual systems. For example, self-cancellation schemes may degrade scheduler performance or may increase the complexity of the algorithms used to schedule different tones to maintain or improve throughput.
Multiplying each OFDM sample in the time domain by a window tap, or windowing, is another common way to shape the spectrum of OFDM signal. Compared to conventional filters and/or self-cancellation schemes for reducing out-of-band emission, windowing is relatively simple to implement at the transmitter. However, windowing typically reduces the orthogonality of the windowed OFDM signals. Accordingly, conventional receivers may have to be modified to compensate for the reduction in the orthogonality of the windowed OFDM signals. For example, the receiver may be required to perform one or more matrix inversions to extract the transmitted signal. The receiver modifications may increase the complexity of the receiver which may increase the cost of the receiver. A raised-cosine window may also be employed for windowing OFDM signals. However, the raised-cosine window is just a common selection and does not guarantee the optimality of the design. Consequently, the overhead associated with the raised-cosine windowing technique may be large, which may reduce the efficiency of the communication system.